The invention relates to a filter useful for separating liquid from solid particles. More particularly, the invention comprises a removable filter which comprises vertically arrayed and manifolded filter sections connected to a common filtrate collector, useful for separating liquid hydrocarbon product from a Fischer-Tropsch hydrocarbon synthesis slurry comprising catalyst particles in a hydrocarbon liquid, and to a hydrocarbon synthesis process employing same. Each filter section comprises a plurality of elongated, vertically disposed and hollow filter elements attached to and horizontally spaced across a manifold, with the manifolds connected to a filtrate conduit for removing the hydrocarbon liquid from the slurry.
Slurry hydrocarbon synthesis (HCS) processes are known. In a slurry HCS process a synthesis gas (syngas) comprising a mixture of H2 and CO is bubbled up as a third phase through a slurry in a reactor in which the slurry liquid comprises hydrocarbon products of the synthesis reaction and the dispersed, suspended solids comprise a suitable Fischer-Tropsch type hydrocarbon synthesis catalyst. Reactors which contain such a three phase slurry are sometimes referred to as xe2x80x9cbubble columnsxe2x80x9d, as is disclosed in U.S. Pat. No. 5,348,982. Irrespective of whether the slurry reactor is operated as a dispersed or slumped bed, the mixing conditions in the slurry will typically be somewhere between the two theoretical conditions of plug flow and back mixed. The catalyst particles are typically kept dispersed and suspended in the liquid by the lifting action of the syngas bubbling up through the slurry and by hydraulic means. Mechanical means such as impellers and propellers and the like are not used, because they will quickly erode and also cause attrition of the catalyst particles. One or more vertical, gas disengaging downcomers may be used as hydraulic means to assist in maintaining more uniform catalyst dispersion, by providing a vertical catalyst circulation in the slurry, as is disclosed in U.S. Pat. No. 5,382,748. The slurry liquid comprises the liquid hydrocarbon products of the HCS reaction and must be separated from the catalyst particles and removed from the reactor for further processing and upgrading. This is typically accomplished by mechanical filtration in which the slurry is fed to one or more filters, either inside the slurry in the reactor or outside the reactor, which permit the liquid to pass through, but not the catalyst particles. U.S. Pat. No. 5,527,473 and patent publications EP 0609079, WO 93/16796 and WO 94/16807 all relate to helically wound wedge wire filters and the like, while WO 93/16795 discloses vertical or helically wound fine metal threads or sintered metal. Magnetically assisted filtration has also been disclosed. Notwithstanding the disclosures of the prior art, there is still a need for an efficient and easily replaceable filter which can be immersed in the slurry.
The present invention relates to a process and filter means for separating particulate solids from a liquid and is useful in a hydrocarbon synthesis (HCS) process. The slurry comprises gas bubbles and particulate solids in a slurry liquid. The filter means comprises a filter which includes at least one filter section which comprises a plurality of hollow filter elements disposed on a manifold connected to a filtrate conduit, for removing liquid from a slurry which comprises particulate solids dispersed in a liquid. In the process of the invention the filter, containing one or more filter sections, is immersed in the slurry, the interior of the filter elements, manifold or manifolds and conduit are all in fluid communication and the filter elements are permeable to the slurry liquid, but not the particulate solids. The filter and process of the invention are useful for separating and removing slurry hydrocarbon liquid from a three phase, Fischer-Tropsch type hydrocarbon synthesis slurry, either inside the slurry reactor or outside the reactor in a separate vessel. In one embodiment, each one or more sections of the filter comprise a plurality of filter elements vertically disposed and horizontally arrayed or arranged across the horizontal surface of a respective manifold which is connected to a common filtrate conduit. The filter elements comprise elongated, hollow tubes or pipes made of a liquid permeable material but impermeable to the passage of the solids therethrough. This is readily achieved by the use of pipes or tubes made of sintered metal. Thus, the wall of the hollow cylindrical elements is permeable to the hydrocarbon liquid, but not the catalyst particles. In a preferred embodiment the filter is vertically and removably secured in the slurry in the slurry HCS reactor (or in an external filtration vessel) by suitable means, such as being suspended from the top of the reactor, so that the filter hangs down in the slurry and can be removed vertically overhead through a nozzle or other orifice in the top of the reactor. This permits facile removal and replacement of the filter, without having to remove the slurry from the reactor or separate filtration vessel. The use of a plurality of liquid permeable filter elements in the form of hollow cylinders, closed at one end with the other end opening into the supporting manifold, enables a very high filtration surface area to be achieved, while occupying minimal space inside the reactor. This can be expressed as the ratio of the filtration area to the volume occupied by the filter in the slurry, or area to volume (A/V) ratio. The filter of the invention will have an A/V of at least 4.3 ftxe2x88x921 and preferably at least 5.9 ftxe2x88x921. Therefore, in one aspect the invention relates to (i) a filter having these A/V ratios and (ii) a slurry reactor and process, and particularly an HCS slurry reactor and process, employing at least one filter having such A/V ratios for separating the slurry liquid from the particulate solids in the slurry, either in the reactor itself or in a separate vessel. Thus, in another embodiment the invention relates to a slurry reactor, and particular slurry HCS reactor, containing at least one filter for separating slurry liquid from solid particles in the slurry, wherein the filter is removably secured in the reactor by means which permit removal of the filter vertically upward and out the top of the reactor. With specific regard to a slurry hydrocarbon synthesis process for forming hydrocarbons, at least a portion of which are liquid, the invention comprises:
(a) reacting a synthesis gas comprising a mixture of H2 and CO in the presence of a solid, particulate hydrocarbon synthesis catalyst in a slurry in a hydrocarbon synthesis reactor at reaction conditions effective to form hydrocarbons, at least a portion of which are liquid at said reaction conditions, wherein said slurry comprises said catalyst and gas bubbles in a hydrocarbon slurry liquid, and wherein said slurry hydrocarbon liquid comprises said liquid hydrocarbons;
(b) contacting said slurry with a filter comprising at least one filter section which includes a plurality of hollow filter elements arrayed across a manifold connected to a filtrate conduit, with the interior of the elements, manifold and conduit in fluid communication, and wherein the wall separating the interior of the elements from the slurry is permeable to said slurry hydrocarbon liquid but not said slurry solids;
(c) passing said slurry hydrocarbon liquid through said filter element walls and into said interior of said elements as a filtrate and then successively passing said filtrate through the interior of said manifold and said filtrate conduit, and
(d) passing said filtrate from said conduit out of said reactor.
The hydrocarbon liquid filtrate removed from the slurry is then typically upgraded to more valuable product by fractionation and/or one or more conversion operations, or sold neat. The slurry or slurry body in which the filter is immersed may be the reactive slurry in the HCS reactor or it may be a slurry body in an external filtration zone. The HCS reactor will typically be operating during filtration and the filtration may be continuous or intermittent. If the HCS reactor is on line and operating to produce hydrocarbons, filtering the hydrocarbon liquids and passing them out of the reactor in the process of the invention does not disturb or interfere with the HCS reactions. In one embodiment in which the filter is immersed in a slurry in a filtration vessel external of the HCS reactor, it is advantageous to feed slurry from the reactor to the external vessel via a solids reducing downcomer, so that the slurry from which the hydrocarbon liquids are being separated from the catalyst particles and withdrawn from the filtration vessel have a reduced solids content compared to the slurry in the HCS reactor. In the practice of the invention in which sintered metal is used as the actual filtration medium, it is possible to have a pore size of less than one micron. This means that essentially no solids (other than extremely small fines) pass through the filtration medium with the hydrocarbon liquid. In yet another embodiment, hydrogen or a hydrogen containing gas is fed into the external filtration unit or vessel to prevent catalyst deactivation. While the process and means of the invention are described herein with particular reference to their usefulness in association with a slurry HCS process, the invention is not intended to be so limited.